In an orthopedic operation, a bone screw is generally used to connect broken bones or to correct various trauma and abnormalities of bones. Since bones are the main force receiving structures by which human bodies bear various mechanical loads, and broken bones that have been connected or the bones that have been corrected are likely to transfer the force loads borne to the bone screws for fixation, thereby in actual use, the bone screw generally needs to bear various kinds of changing forces in different frequency. This not only requires that the bone screw itself should be very strong so as not to be damaged under an action of an external force, but also requires that the bone screw should be fixedly engageable with bone tissues so as not to escape from the bones under external force.
With respect to the first requirement, the main representation is that the bone screw has a comparatively high bending strength and will not break under an action of an external force perpendicular to the direction of the axis. With respect to the second requirement, the main representation is that the bone screw not only will not escape under axial pullout force, but also can hardly rotate along an axis under moment.
In order to increase the capability of resisting the axial pullout force and an axial turning moment of the bone screw, a traditional method is increasing the height of the tooth form of the threads and increasing the density of the threads. This will significantly increase the contact surface area of the bone screw and the bone to thereby significantly increase the force receiving areas of the axial pullout force and the axial turning moment, thereby improving the axial pullout force and the axial turning moment that can be borne by the bone screw. However, such a method may cause adverse effects on the other mechanical properties of the bone screw.
For example, with regard to the bending strength of the screw, the increase of the height of the tooth form of the threads may make the inner diameter of the thread decrease relatively, which makes the cross section area of the screw capable of bearing a bending stress decrease and the bending strength decrease significantly. In particular, when the density of the threads is comparatively high, a sharp bending angle is likely to be formed at the root, which results in stress concentration and further reduces the bending strength of the screw. Besides, the height of the tooth form being too high and the axial size of the tooth form being too small will result in break of a thread tooth.
On the other hand, in order to maintain a comparatively high bending strength, the inner diameter of the threads has to be increased to thereby reduce the height of the tooth form of the threads, and the result is that dense threads having a low height of the tooth form are formed on the surface of the screw. Obviously, the capacity of resisting the axial pullout force and the axial turning moment of such a screw structure will not be too high, for the contact area of the screw and the bone is too small.
Thus, people urgently need a new bone screw, which can have the improved characteristics of resisting three external forces, i.e., the axial pullout force, the axial turning moment and the bending moment, while being contacted and engaged with the bone, thereby improving the property of the bone screw entirely.